Projection optical system, exposure apparatus and device fabricating method

ABSTRACT

A projection optical system is provided for projecting a pattern on an object surface onto an image surface in a reduced size. The projection optical system has six reflective surfaces that include, in order of reflecting light from the object surface, a first reflective surface, a second convex reflective surface, a third convex reflective surface, a fourth reflective surface, a fifth reflective surface and a sixth reflective surface. An aperture stop is provided along an optical path between the first and second reflective surfaces. The following condition is met by the system, where L 1  is an interval between the object surface and the surface apex that is the closest to the object surface, and L 2  is an interval between the surface apex of the first reflective surface and the surface apex that is the closest to the object surface: 
     
       
         
           
             0.75 
             &lt; 
             
               L1 
               L2 
             
             &lt; 
             
               1.25 
               .

BACKGROUND OF THE INVENTION

The present invention relates generally to exposure apparatuses, andmore particularly to a catoptric projection optical system, an exposureapparatus, and a device fabricating method using the same. The catoptricprojection optical system use ultraviolet (“UV”) and extreme ultraviolet(“EUV”) light to project and expose an object, such as a single crystalsubstrate for a semiconductor wafer, and a glass plate for a liquidcrystal display (“LCD”).

Along with recent demands for smaller and lower profile electronicdevices, finer semiconductor devices to be mounted onto these electronicdevices have been increasingly demanded. For example, the design rulefor mask patterns has required that an image with a size of a line andspace (“L & S”) of less than 0.1 μm be extensively formed and it isexpected to require circuit patterns of less than 80 nm in the nearfuture. The L & S denotes an image projected onto a wafer in exposurewith equal line and space widths, and serves as an index of exposureresolution.

A projection exposure apparatus as a typical exposure apparatus forfabricating semiconductor devices includes a projection optical systemfor projecting and exposing a pattern on a mask or a reticle (theseterms are used interchangeably in the present application), onto awafer. The resolution R of the projection exposure apparatus (i.e. aminimum size for a precise image transfer) can be defined using alight-source wavelength λ and the numerical aperture (“NA”) of theprojection optical system as in the following Equation:

$R = {k_{1} \times \frac{\lambda}{NA}}$

As the shorter the wavelength becomes and the higher the NA increases,the better the resolution becomes. The recent trend has required thatthe resolution be a smaller value; however it is difficult to meet thisrequirement using only the increased NA, and the improved resolutionexpects use of a shortened wavelength. Exposure light sources havecurrently been in transition from KrF excimer laser (with a wavelengthof approximately 248 nm) and ArF excimer laser (with a wavelength ofapproximately 193 nm) to F2 excimer laser (with a wavelength ofapproximately 157 nm). Practical use of the EUV light is being promotedas a light source.

As a shorter wavelength of light limits usable glass materials fortransmitting the light, it is advantageous for the projection opticalsystem to use reflection elements, i.e., mirrors instead of using manyrefraction elements, i.e., lenses. No applicable glass materials havebeen proposed for the EUV light as exposure light, and a projectionoptical system could not include any lenses. It has thus been proposedto form a catoptric reduction projection optical system only withmirrors.

A mirror in a catoptric reduction projection optical system forms amultilayer coating to enhance reflected light and increase reflectance,but the smaller number of mirrors is desirable to increase reflectanceof the entire optical system. In addition, the projection optical systempreferably uses the even number of mirrors to avoid mechanicalinterference between the mask and the wafer by arranging the mask andthe wafer at opposite sides with respect to a pupil. A smaller criticaldimension (or resolution) for the EUV exposure apparatus than aconventional one requires a large NA (e.g., NA of 0.2 for a wavelengthof 13.5 nm), while it is hard for the conventional 3 to 4 mirrors todecrease the wave aberration. For the increased degree of freedom incorrecting the wave aberration, the increased number of mirrors isneeded as well as making the mirrors aspheric. As a result, theprojection optical system comes to require so many as six mirrors (whilethe instant application calls such an optical system a six-mirrorsystem). Such six-mirror systems are disclosed, for example, in U.S.Pat. No. 6,033,079 and WO 02/48796.

U.S. Pat. No. 6,033,079 discloses two typical six-mirror catoptric, EUVprojection optical systems in its embodiments. These projection opticalsystems receive light from an object plane, form an intermediate imagevia four mirrors, i.e., a concave first reflective surface, a concave orconvex second reflective surface, a convex third reflective surface, anda concave fourth reflective surface, and re-form the intermediate imageon an image plane via a convex fifth reflective surface and a concavesixth reflective surface. Both of these two embodiments arrange anaperture stop on the second reflective surface.

International Patent Publication No. WO 02/48796 discloses three typicalsix-mirror catoptric, EUV projection optical systems in its embodiments.These projection optical systems receive light from an object plane,form an intermediate image via a concave first reflective surface and aconcave second reflective surface, and re-form the intermediate image onan image plane via a convex third reflective surface, a concave fourthreflective surface, a convex fifth reflective surface and a concavesixth reflective surface. Each of these three embodiments provides anaperture stop between the first and second reflective surfaces along theoptical axis.

Other prior art that disclose similar optical systems include JapanesePatent Applications, Publication Nos. 2003-15040, 2001-185480,2002-6221, U.S. Patent Application, Publication No. 2003/0076483, andU.S. Pat. No. 6,172,825.

However, the configurations disclosed in U.S. Pat. No. 6,033,079 aredisadvantageous, because the aperture stop is located on the secondreflective surface and causes an increased effective diameter of thefourth reflective surface. More specifically, the EUV projection opticalsystem applies a multilayer coating on a mirror surface so as toincrease the reflectance, and the reduced incident angle of a ray, i.e.,an angle between the ray and a normal of the reflective surface, issuitable for characteristics of the multilayer coating. Since the EUVprojection optical system increases an effective diameter of the sixthreflective surface in order to increase the NA and improve theresolution, the fourth reflective surface should be located apart fromthe optical axis in order to prevent light shielding. Since theembodiments in U.S. Pat. No. 6,033,079 arrange the aperture stop on thesecond reflective surface, only the third reflective surface introducesthe light to the fourth reflective surface apart from the optical axis.In order to reduce the incident angle as discussed, a distance betweenthe third and fourth reflective surfaces should be made large. Becauseof this large distance, a distance between the object and the fourthreflective surfaces becomes small, so it is hard to make a space forplacing a mirror or something. And this increases the light's divergenceonto the fourth reflective surface, requires the extremely large maximumeffective diameter of 700 mm, and causes a system whose accuracy offinishing is hard to measure.

The configurations described in WO 02/48796 use a concave shape for boththe first and second reflective surfaces, and tend to condense the lighton a surface close to the object plane. Therefore, the intermediateimage is formed near the third reflective surface, and the divergence onthe third reflective surface. Then disadvantageously, ripples on amirror surface, which are formed during a processing operation and airbubbles in the mirror material directly deteriorate imaging performance.In addition, the temperature rise in the reflective surface due to theenergy concentration deforms the mirror shape, and dust on the mirrorsurface is transferred onto the wafer. Moreover, while the second andthird reflective surfaces introduce the light into the fourth reflectivesurface apart from the optical axis from the aperture stop, the lightfrom the second reflective surface to the third reflective surfaceapproaches to the optical axis due to the concave shape of the secondreflective surface and is hard to introduce into the fourth reflectivesurface apart from the optical axis. To solve the problem, the distancebetween the first and the second reflective surfaces should be madelarge, so the distance between the object plane and the second surfacebecomes small. Therefore it is hard to make a space for placing a mirroror something.

BRIEF SUMMARY OF THE INVENTION

A projection optical system of one aspect according to the presentinvention for projecting a pattern on an object plane onto an imageplane in a reduced size, said projection optical system includes sixreflective surfaces that include, in order of reflecting light from theobject plane, a first reflective surface, a second convex reflectivesurface, a third convex reflective surface, a fourth reflective surface,a fifth reflective surface and a sixth reflective surface, and anaperture stop along an optical path between the first and secondreflective surfaces, wherein the six reflective surfaces are arranged sothat radii of curvature of these six reflective surfaces aresubstantially aligned with a certain optical axis, wherein a surfaceapex is defined, with respect to each of the six reflective surfaces, asone of nodal points between an optical axis and an approximatelyspherical surface, which one is closest to a reflection point of thelight on each of the six reflective surfaces, the approximatelyspherical surface having a center that is defined as a center ofcurvature of each of the six reflective surfaces, and a radius that isdefined as a radius of curvature of each of the six reflective surfaces,and wherein the following condition is met, where L1 is an intervalbetween the object plane and the surface apex that is the closest to theobject plane (or a distance between the object plane and a surface apexthat is the closest to the object plane among six surface apexes of thesix reflective surfaces), and L2 is an interval between the surface apexof the first reflective surface and the surface apex that is the closestto the object plane:

$0.75 < \frac{L1}{L2} < {1.25.}$

A projection optical system of another aspect according to the presentinvention for projecting a pattern on an object plane onto an imageplane in a reduced size, said projection optical system comprising sixreflective surfaces that include, in order of reflecting light from theobject plane, a first reflective surface, a second reflective surface, athird convex reflective surface, a fourth reflective surface, a fifthreflective surface and a sixth reflective surface, wherein saidprojection optical system forms an intermediate image of the pattern,wherein the six reflective surfaces are arranged so that radii ofcurvature of these six reflective surfaces are substantially alignedwith a certain optical axis, wherein a surface apex is defined, withrespect to each of the six reflective surfaces, as one of nodal pointsbetween an optical axis and an approximately spherical surface, whichone is closest to a reflection point of the light on each of the sixreflective surfaces, the approximately spherical surface having a centerthat is defined as a center of curvature of each of the six reflectivesurfaces, and a radius that is defined as a radius of curvature of eachof the six reflective surfaces, and wherein the following condition ismet, where L1 is an interval between the object plane and the surfaceapex that is the closest to the object plane (or a distance between theobject plane and a surface apex that is the closest to the object planeamong six surface apexes of the six reflective surfaces), and L2 is aninterval between the surface apex of the first reflective surface andthe surface apex that is the closest to the object plane:

$0.75 < \frac{L1}{L2} < {1.25.}$

An exposure apparatus according to another aspect of the presentinvention includes an illumination optical system for illuminating apattern on an object plane using light from a light source, and theprojection optical system for projecting the pattern on the object planeonto an image plane in a reduced size.

A device fabricating method according to still another aspect of thepresent invention includes the steps of exposing an object to be exposedusing the above exposure apparatus, and developing the exposed object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view according to a first embodiment of the presentinvention.

FIG. 2 is a view according to a second embodiment of the presentinvention.

FIG. 3 is a view according to a third embodiment of the presentinvention.

FIG. 4 is a view according to a fourth embodiment of the presentinvention.

FIG. 5 is a view according to a fifth embodiment of the presentinvention.

FIG. 6 is a view according to a sixth embodiment of the presentinvention.

FIG. 7 is a schematic structure of an exposure apparatus according toone embodiment of the present invention.

FIG. 8 is a flowchart for explaining a method for fabricating devices(semiconductor chips such as ICs, LSIs, and the like, LCDs, CCDs, etc.).

FIG. 9 is a detailed flowchart for Step 4 of wafer process shown in FIG.8.

FIG. 10 is a view showing Lmin and Lmax.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A projection optical system of one aspect according to the presentinvention for projecting a pattern on an object plane onto an imageplane in a reduced size, said projection optical system includes sixreflective surfaces that include, in order of reflecting light from theobject plane, a first reflective surface, a second convex reflectivesurface, a third convex reflective surface, a fourth reflective surface,a fifth reflective surface and a sixth reflective surface, and anaperture stop along an optical path between the first and secondreflective surfaces, wherein the six reflective surfaces are arranged sothat radii of curvature of these six reflective surfaces aresubstantially aligned with a certain optical axis, wherein a surfaceapex is defined, with respect to each of the six reflective surfaces, asone of nodal points between an optical axis and an approximatelyspherical surface, which one is closest to a reflection point of thelight on each of the six reflective surfaces, the approximatelyspherical surface having a center that is defined as a center ofcurvature of each of the six reflective surfaces, and a radius that isdefined as a radius of curvature of each of the six reflective surfaces,and wherein the following condition is met, where L1 is an intervalbetween the object plane and the surface apex that is the closest to theobject plane (or a distance between the object plane and a surface apexthat is the closest to the object plane among six surface apexes of thesix reflective surfaces), and L2 is an interval between the surface apexof the first reflective surface and the surface apex that is the closestto the object plane:

$0.75 < \frac{L1}{L2} < {1.25.}$

A projection optical system of another aspect according to the presentinvention for projecting a pattern on an object plane onto an imageplane in a reduced size, said projection optical system comprising sixreflective surfaces that include, in order of reflecting light from theobject plane, a first reflective surface, a second reflective surface, athird convex reflective surface, a fourth reflective surface, a fifthreflective surface and a sixth reflective surface, wherein saidprojection optical system forms an intermediate image of the pattern,wherein the six reflective surfaces are arranged so that radii ofcurvature of these six reflective surfaces are substantially alignedwith a certain optical axis, wherein a surface apex is defined, withrespect to each of the six reflective surfaces, as one of nodal pointsbetween an optical axis and an approximately spherical surface, whichone is closest to a reflection point of the light on each of the sixreflective surfaces, the approximately spherical surface having a centerthat is defined as a center of curvature of each of the six reflectivesurfaces, and a radius that is defined as a radius of curvature of eachof the six reflective surfaces, and wherein the following condition ismet, where L1 is an interval between the object plane and the surfaceapex that is the closest to the object plane (or a distance between theobject plane and a surface apex that is the closest to the object planeamong six surface apexes of the six reflective surfaces), and L2 is aninterval between the surface apex of the first reflective surface andthe surface apex that is the closest to the object plane:

$0.75 < \frac{L1}{L2} < {1.25.}$

It is preferable that the first reflective surface has a concave shape.It is also preferable that the fourth reflective surface has a concaveshape. It is also preferable that an intermediate image is formed alongthe optical path from the fourth reflective surface to the fifthreflective surfaces.

Preferably, 25°<θ_(max)+Δθ<35° is met, where θ_(max) is a maximumincident angle on one of the six reflective surfaces, which one has thelargest maximum value of a light incident angle, and Δθ is an incidentangle distribution width on one of the six reflective surfaces, whichone has the largest maximum value of a light incident angle. Morepreferably, 28°<θ_(max)+Δθ and/or θ_(max)+Δθ<32° or θ_(max)+Δθ<30° aremet.

Moreover, the six reflective surfaces are arranged so that radii ofcurvature of these six reflective surfaces are substantially alignedwith a certain optical axis. Suppose that a surface apex is defined,with respect to each of the six reflective surfaces, as one of nodalpoints between an optical axis and an approximately spherical surface,which one is closest to a reflection point of the light on each of thesix reflective surfaces. The approximately spherical surface has acenter that is defined as a center of curvature of each of the sixreflective surfaces, and a radius that is defined as a radius ofcurvature of each of the six reflective surfaces. It is preferable thatthe following condition is met, where L1 is an interval between theobject plane (which, as used herein, means a surface on which the objectis located) and the surface apex that is the closest to the object plane(although L1 is more specifically is a distance between a nodal pointbetween the object plane and the optical axis and the surface apex thatis the closest to the object plane), and L2 is an interval between thesurface apex of the first reflective surface and the surface apex thatis the closest to the object plane:

$0.9 < \frac{L1}{L2} < 1.1$

It is also preferable that none of the six reflective surfaces have anarea that allows the light to pass through, absorbs the light ortransmits the light, in their light incident areas. Preferably, anoptical path from the second reflective surface to the third reflectivesurface does not intersect with an optical path from the fourthreflective surface to the fifth reflective surface. Preferably, the sixreflective surfaces are arranged so that radii of curvature of these sixreflective surfaces are substantially aligned with a certain opticalaxis.

Preferably, there is an extreme of a maximum incident angle at eachpoint on a nodal line in a region between Lmin+0.3×(Lmax−Lmin) and Lmaxon the nodal line, the nodal line being formed between a light incidentarea upon which the light from an arc illuminated area on the objectplane is incident, and a plane that includes a center point of a chordof the arc illuminated area shape on the object plane and an opticalaxis, the light incident area being located on one of the six reflectivesurfaces, which one has the largest maximum value of a light incidentangle, where Lmin is a minimum distance from the optical axis to eachpoint on the nodal line, and Lmax is a maximum distance from the opticalaxis to each point on the nodal line. It is preferable that the maximumincident angle at each point has an extreme value on the nodal line in aregion between (Lmin+Lmin)/2+0.2×(Lmax−Lmin) on the nodal line.

It is preferable that the third reflective surface is one of the sixreflective surfaces, which one has the maximum light incident angle.

It is preferable that one of the six reflective surfaces, which one hasthe largest maximum value of a light incident angle, has a convex shapefor receiving convergent light and reflects divergent light.

Preferably, a difference is 30 mm or longer, more preferably 40 mm orlonger, between a maximum distance and a minimum distance between anoptical axis and a light incident area on the fourth reflective surface,upon which light emitted from a center of a nodal line is incident. Thenodal line is formed between a plane that includes a center of an arcilluminated area (i.e., a center of gravity) on the object plane and theoptical axis, and the illuminated area.

If it is assumed that a surface apex is defined, with respect to each ofthe six reflective surfaces, as one of nodal points between an opticalaxis and an approximately spherical surface, which one is closest to areflection point of the light on each of the six reflective surfaces. Inaddition, the approximately spherical surface has a center that isdefined as a center of curvature of each of the six reflective surfaces(which means a center of curvature of the reflective surface when thereflective surface is a spherical surface and a center of curvature of aspherical surface after the aspheric component is removed from thereflective surface if the reflective surface is an aspheric surface),and a radius that is defined as a radius of curvature of each of the sixreflective surfaces (which means a radius of curvature of asphericalsurface after the aspheric component is removed from each reflectivesurface if each reflective surface is an aspheric surface). Thenpreferably, the surface apexes of the six reflective surfaces arearranged in order of the fourth reflective surface, the secondreflective surface, the third reflective surface, the first reflectivesurface, the sixth reflective surface and the fifth reflective surfacein order from the object plane to the image plane along an optical axis.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, theapproximately spherical surface having a center that is defined as acenter of curvature of each of the six reflective surfaces, and a radiusthat is defined as a radius of curvature of each of the six reflectivesurfaces, wherein a distance is 250 mm or longer, more preferably 310 mmor longer, between the object plane and one of the surface apexes of thesix reflective surfaces, which one is the closest to the object plane.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, a distancealong the optical axis between the surface apex of the second reflectivesurface and the surface apex of the fourth reflective surface is 5 mm orlonger, preferably 10 mm or longer, more preferably 15 mm or longer.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, theapproximately spherical surface having a center that is defined as acenter of curvature of each of the six reflective surfaces, and a radiusthat is defined as a radius of curvature of each of the six reflectivesurfaces, wherein Lall/200<L24<Lall/10, more preferably, Lall/100<L24and/or L24<Lall/18, are met, where L24 is an interval between thesurface apex of the second reflective surface and the surface apex ofthe fourth reflective surface, and Lall is a distance between the objectplane and the image plane along the optical axis.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, theapproximately spherical surface having a center that is defined as acenter of curvature of each of the six reflective surfaces, and a radiusthat is defined as a radius of curvature of each of the six reflectivesurfaces, wherein an interval between the surface apex of the sixthreflective surface and the closest surface apex of the sixth reflectivesurface along the optical axis is 100 mm or longer. Preferably, theinterval between the surface apex of the six reflective surface and theclosest surface apex of the reflective surface along the optical axis is110 mm or longer, more preferably, 115 mm or longer.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, theapproximately spherical surface having a center that is defined as acenter of curvature of each of the six reflective surfaces, and a radiusthat is defined as a radius of curvature of each of the six reflectivesurfaces, wherein Lall/20<L6<Lall/6, more preferably, Lall/12<L6 and/orL6<Lall/9, are met, where L6 is an interval between the surface apex ofthe sixth reflective surface and the surface apex of the reflectivesurface that is closest to the sixth reflective surface, and Lall is adistance between the object plane and the image plane along the opticalaxis.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, theapproximately spherical surface having a center that is defined as acenter of curvature of each of the six reflective surfaces, and a radiusthat is defined as a radius of curvature of each of the six reflectivesurfaces, wherein the surface apex of the third reflective surface islocated closer to the object plane than the surface apex of the sixthreflective surface.

Preferably, the third reflective surface is located closer to the objectplane along the optical path than the sixth reflective surface.

Preferably, an absolute value of a radius of curvature of the secondreflective surface is 1800 mm or smaller, more preferably 1600 mm orsmaller.

Preferably, an intermediate image of the pattern is formed at a positionthat accords with none of the six reflective surfaces.

Preferably, an intermediate image of the pattern on the object plane isformed between two adjacent reflective surfaces along the optical pathof the light among the six reflective surfaces, and located apart fromeach of the two adjacent reflective surfaces by Lim×0.35 or greater,more preferably, Lim×0.4 or greater, where Lim is an optical path lengthbetween the two adjacent reflective surfaces.

Preferably, a position of the aperture stop is the closest to the secondreflective surface among the six reflective surfaces with respect to adistance along the optical path.

Preferably, the aperture stop is located between the first and secondreflective surfaces, and located apart from each of the first and secondreflective surfaces by Lst/10 or greater, more preferably, by Lst/5 orgreater, where Lst is an optical path length between the first andsecond reflective surfaces.

Preferably, the aperture stop is located on the optical path between thefirst and second reflective surfaces.

Preferably, the six reflective surfaces are located between the objectplane and the image plane (or the six reflective surfaces are locatedbetween the object plane or the object-side plane including this objectplane and the image plane of the image-side plane including this imageplane).

Preferably, all the optical elements having optical powers are locatedbetween the object plane and the image plane in the catoptric projectionoptical system.

Preferably, at least one of the six reflective surfaces is an asphericmirror having a multilayer coating for reflecting the EUV light.

Preferably, all of the six reflective surfaces are aspheric mirrors eachhaving a multilayer coating for reflecting the EUV light.

Preferably, the light used for the projection optical system is EUVlight with a wavelength between the 10 nm and 20 nm, more preferably 13nm and 14 nm.

Preferably, the optical system at the object plane side isnon-telecentric. More preferably, the optical system at the image planeside is substantially telecentric.

An exposure apparatus of the instant embodiments includes anillumination optical system for illuminating a pattern on an objectplane using light from a light source, and the above one of projectionoptical systems for projecting the pattern on the object plane onto animage plane in a reduced size. A reflection mask is preferably locatedon the object plane. In addition, it preferably further includes meansfor synchronously scanning a mask stage for supporting the object planeand a wafer stage for supporting the image plane, while the object planeis illuminated by the EUV light.

A device fabricating method of one embodiment according to the presentinvention includes the steps of exposing an object using the aboveexposure apparatus, and developing the object that has been exposed.

With the forgoing in mind, a description will now be given of acatoptric reduction projection optical system according to one aspect ofthe present invention, with reference to the accompanying drawings.However, the present invention is not limited to these embodiments, andeach element is replaceable with another element within a scope thatachieves an object of the present invention.

Here, FIGS. 1 and 3 are sectional views of the inventive catoptricreduction projection optical system and its optical path. The inventivecatoptric reduction projection optical system is a catoptric reductionprojection optical system for projects a pattern on an object plane MS(for example, a mask surface) onto an image plane W (for example, asurface of an object to be exposed, such as a substrate) in a reducedsize, and is suitable for the EUV light (with a wavelength between 10 nmand 15 nm, more preferably between 13.4 nm and 13.5 nm).

The catoptric reduction projection optical system includes six mirrors,which basically includes, in order of reflecting light from the objectplane MS side, a first (concave) mirror M1, a second (convex) mirror M2,a third (convex) mirror M3, a fourth (concave) mirror M4, and a fifth(convex) mirror M5, and a sixth (concave) mirror M6, and arranges anaperture stop between the first and second reflective surfaces. A linethat connects each center of curvature in these six reflective surfacesof these six mirrors is referred to as an optical axis. However, thecenters of curvatures in the six mirrors do not always align with aline, and it is conceivable that the center of curvature of a certainmirror offsets from the optical axis slightly (or within 1% or smallerof the mirror's radius of curvature) for purposes of aberrationalcorrections, etc. The mirror's center of curvature means a center ofcurvature of a spherical surface as a base of an aspheric surface if themirror is not spherical but aspheric. Similarly, the mirror's radius ofcurvature means a radius of curvature of a spherical surface as a baseof an aspheric surface if the mirror is not spherical but aspheric.

An optical path of the light from a reticle's pattern surface objectplane) to the image plane is configured so that an optical path from thesecond reflective surface to the third reflective surface does notintersect with an optical path from the fourth reflective surface to thefifth reflective surface. Therefore, a complex arrangement of opticalelements and two or more surfaces with large effective diameter can beavoided. This prevents an optical path from being so complex that it isdifficult to arrange members, or two surface surfaces or more eachhaving a large effective diameter from existing.

For easy processing and measurements, an absolute value of a radius ofcurvature of the second reflective surface is made 1800 mm or smaller,more preferably 1600 mm or smaller.

An intermediate image is formed between the fourth and fifth reflectivesurfaces. This configuration narrows the light near the sixth reflectivesurface having a large effective diameter, and effectively preventslight shielding. An intermediate image of the pattern is formed at aposition that accords with none of the six reflective surfaces. Anintermediate image of the pattern on the object plane is formed betweentwo adjacent reflective surfaces along the optical path of the lightamong the six reflective surfaces, and the intermediate image is locatedapart from each of the two adjacent reflective surfaces by Lim×0.35 orgreater, more preferably, Lim×0.4 or greater, where Lim is an opticalpath length between the two adjacent reflective surfaces. Thisconfiguration can maintain an appropriate divergence of the light on thereflective surface, and reduce deteriorations of imaging performance onthe reflective surface, caused by dust and ripples, bubbles in themirror material, etc.

The six reflective surfaces, such as the first reflective surface, thesecond reflective surface, the third reflective surface, the fourthreflective surface, the fifth reflective surface and the sixthreflective surface, are arranged so that radii of curvature of these sixreflective surfaces are substantially aligned with a certain opticalaxis. A center of curvature, as used herein, means a center of curvatureof the reflective surface when the reflective surface is substantially aspherical surface, and a center of curvature of a spherical surfaceafter an aspheric component is removed from the reflective surface ifthe reflective surface is the aspheric surface. In other words, thecenter of curvature means one based on the curvature near a rotationallycenter axis of the reflective surface (which means any one of lines thatpass through a center of a spherical surface when the reflective surfaceis the spherical surface, or a rotationally center axis of arotationally symmetrical aspheric surface that includes the reflectivesurface when the reflective surface is an aspheric surface).

Characteristically, the light from the object plane MS to the firstmirror M1 is non-telecentric, and the exit light at the image plane sideis telecentric. Since an additionally provided illumination opticalsystem illuminates the reflection mask on the object plane MS, a certainincident angle is vital at the object side. On the other hand, the imageplane side is preferably telecentric to reduce magnification varianceeven when the wafer W arranged on the image plane moves along theoptical axis direction.

The inventive catoptric reduction projection optical system is arrangedsubstantially as a coaxial optical system that is axially symmetricalaround one optical axis, has an advantage in that an aberration iscorrected for only an arc-shape image field around the optical axis.However, the six mirrors in the catoptric reduction projection opticalsystem do not have to be arranged perfectly coaxial for aberrationalcorrections or adjustments. For example, they may slightly decenter foraberrational improvements or improve the degree of freedom inarrangement.

Preferably, the fifth and sixth mirrors (M5) and (M6) are convex andconcave mirrors, respectively, for imaging with high NA and maintainedback focus. Here, the “back focus” means an interval between the surfaceclosest to the image plane and the image plane (W). It is preferablethat the first reflective surface has a concave shape for convergence ofthe divergent light from the mask, therefore it is easy to guide thelight from the mask to the wafer. It is also preferable that the fourthreflective surface has a concave shape, so as to introduce the lightinto the fifth reflective surface close to the optical axis and avoidthe sixth reflective surface having a large effective diameter.

In addition, all the reflective surfaces are arranged between the objectplane and the image plane, and the reticle stage and wafer stage can bearranged easily.

In general, the fourth reflective surface should be arranged at aposition apart from the optical axis so as to avoid light shielding atthe sixth reflective surface having a great diameter. In the instantembodiment, the aperture stop is located between the first and secondreflective surfaces, and the second reflective surface and the thirdreflective surface have a convex shape. This configuration can introducethe light to the fourth reflective surface via two reflective surfaces,i.e., the second and third reflective surfaces, when attempting tointroduce the light from the aperture stop to the fourth reflectivesurface, preventing an extremely great distance between the third andfourth reflective surfaces. This provides a system that has a relativelysmall maximum effective diameter, and can reduce the enlargement of thefourth reflective surface while keeping the incident angle small.Moreover, the properly enlarged front focus (that is a distance from theobject plane to the fourth reflective surface in the instant embodiment)facilitates an arrangement of optical elements.

In order to form a system having a comparatively small incident angle, acomparatively small maximum effective diameter, and a sufficient frontfocus, it is preferable that 0.75<L1/L2<1.25, more preferably0.90<L1/L2<1.1, are met, where L1 is an interval between the objectplane and the reflective surface closet to the object plane (which is,but not limited to, a distance between the object plane and the fourthreflective surface in the instant embodiment, and may be a distancebetween the objective plane and the second reflective surface other thanthe second and fourth reflective surfaces), and L2 is an intervalbetween the reflective surface closest to the object plane and the firstreflective surface.

A small telecentricity at the object side is needed to eliminate aproblem of deteriorations of imaging performance caused by influence ofthe oblique incidence upon the reticle. In this case, when the secondreflective surface accords with the aperture stop surface, the lightincident upon the first reflective surface from the object plane isshielded on the second reflective surface. The instant system arrangesthe stop between the first and second reflective surfaces, and preventsthe above problem.

Disadvantageously, when the divergence of the light on the reflectivesurface is small, the ripples on the mirror surface that occur at thetime of processing and the air bubbles in the mirror materials directlyaffect the deteriorations of the imaging performance. In addition, theenergy concentration deforms a mirror and transfers dust. As onesolution for this problem, it is preferable that a difference is 30 mmor longer, more preferably, 40 mm or above (although an extremely greatvalue makes a processing measurement impossible), between a maximumdistance and a minimum distance between an optical axis and a lightincident area on the fourth reflective surface, upon which light emittedfrom a center of a nodal line is incident, the nodal line being formedbetween a plane that includes a center of an arc illuminated area (i.e.,a center of gravity or a center point on the center line in acircumference direction of the arc shape) on the object plane and theoptical axis, and the illuminated area. While the instant systemrelatively reduces a distance between the third and fourth reflectivesurfaces as discussed above and narrows the divergent light on thefourth reflective surface, the convex shapes of the second and thirdreflective surfaces enable properly spread light to be incident uponeach reflective surface and maintains an appropriate divergence of thelight on the fourth reflective surface.

Where r1 to r6 are radii of curvature of respective mirrors, the sum ofPetzval terms should be zero or nearly zero as in Equations below:

${\frac{1}{r_{1}} - \frac{1}{r_{2}} + \frac{1}{r_{3}} - \frac{1}{r_{4}} + \frac{1}{r_{5}} - \frac{1}{r_{6}}} \approx 0$${\frac{1}{r_{1}} - \frac{1}{r_{2}} + \frac{1}{r_{3}} - \frac{1}{r_{4}} + \frac{1}{r_{5}} - \frac{1}{r_{6}}} = 0$

While the inventive catoptric reduction projection optical systemincludes six mirrors, at least one or more mirrors may have an asphericsurface. Equation below defines a general aspheric shape. As a mirrorhaving an aspheric surface advantageously facilitates a correction ofaberration, the aspheric surface is preferably applied to many possible(desirably, six) mirrors.

$Z = {\frac{{ch}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}h^{2}}}} + {Ah}^{4} + {Bh}^{6} + {Ch}^{8} + \mspace{40mu}{Dh}^{10} + {Eh}^{12} + {Fh}^{14} + {Gh}^{16} + {Hh}^{18} + {Jh}^{20} + \ldots}$where “Z” is a coordinate in an optical-axis direction, “c” is acurvature (i.e., a reciprocal number of the radius r of curvature), “h”is a height from the optical axis, “k” a conic constant, “A” to “J” areaspheric coefficients of 4^(th) order, 6^(th) order, 8^(th) order,10^(th) order, 12^(th) order, 14^(th) order, 16^(th) order, 18^(th)order, 20^(th) order, respectively.

A multilayer coating for reflecting the EUV light is applied to therespective mirrors, and intensifies the light. An applicable multilayerfor reflecting the EUV light below 20 nm is, for example, a Mo/Simultilayer coating created by reciprocally laminating a molybdenum (Mo)layer and a silicon (Si) layer or a Mo/Be multilayer coating created byreciprocally laminating a molybdenum (Mo) layer and a beryllium (Be)layer. An optimal material is selected according to wavelengths to beused. Of course, the present invention does not limit the multilayercoating to the above materials, and may use any multilayer coating thathas an operation or effect similar to that of the above.

In general, in view of the characteristics of the multilayer coating, arelatively small distribution width of an incident angle is needed toincrease the reflectance when the maximum value of the incident angle islarge, although a relatively large distribution width of an incidentangle is permissible when the maximum value of the incident angle issmall. It is the third surface that has the largest incident angle amongthe six reflective surfaces in the instant system. This third reflectivesurface is likely to reduce the reflectance irrespective of a relativelysmall incident angle. Considering the characteristics of the multilayercoating, the instant system prevents the deterioration of theperformance by requiring the incident angle characteristic upon thethird reflective surface to satisfy the following the followingequations:25°<θ_(max)+Δθ<35°where θ_(max) is a maximum incident angle upon the third reflectivesurface, and Δθ is a difference between the maximum incident angle andthe minimum incident angle on the third reflective surface, or anincident angle distribution width. More preferably, 28°<θ_(max)+Δθand/or θ_(max)+Δθ<32° or θ_(max)+Δθ<30° are met.

None of the six reflective surfaces have an area that allows the lightto pass though, absorbs the light or transmits the light, in their lightincident areas. In one embodiment, the light incident area on eachreflective surface does not have an opening, etc. With respect to thelight to the object to be exposed (i.e., a wafer), or the light from thesixth reflective surface to the object image plane), an area that doesnot include the light is eliminated from an outer circumference on asection orthogonal to the optical axis. In general, when a pupil isshielded, the imaging performance is remarkably affected. Thisconfiguration overcomes this problem.

The convergent light enters the convex third reflective surface from thesecond reflective surface, and the divergent light enters the fourthreflective surface from the convex third reflective surface. The instantembodiment provides such characteristics that the maximum incident anglein a light incident effective area at respective points in a radialdirection (orthogonal to the optical axis) on the third reflectivesurface has a maximum value at the effective region, thereby making theincident angle distribution width relatively small and preventingdeteriorations of the performance of the multilayer coating. Morespecifically, on the third reflective surface, there is an extreme of amaximum incident angle at each point on a nodal line in a region betweenLmin+0.3×(Lmax−Lmin) and Lmax on the nodal line, the nodal line beingformed between a light incident area upon which the light from an arcilluminated area on the object plane is incident, and a plane thatincludes a center point of a chord of the arc illuminated area shape onthe object plane and an optical axis, the light incident area beinglocated on one of the six reflective surfaces, which one has the largestmaximum value of a light incident angle, where Lmin is a minimumdistance from the optical axis to each point on the nodal line, and Lmaxis a maximum distance from the optical axis to each point on the nodalline. It is more preferable that the maximum incident angle at eachpoint has an extreme value on the nodal line in a region between(Lmin+Lmin)/2.+−.0.2×(Lmax−Lmin) on the nodal line.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, theapproximately spherical surface having a center that is defined as acenter of curvature of each of the six reflective surfaces (which meansa center of curvature of the reflective surface when the reflectivesurface is a spherical surface, and a center of curvature of a sphericalsurface after the aspheric component is removed from the reflectivesurface if the reflective surface is an aspheric surface or a center ofcurvature based on the curvature near the rotationally symmetrical axisof the rotationally symmetrical aspheric surface that includes theaspheric surface), and a radius that is defined as a radius of curvatureof each of the six reflective surfaces (which means a radius ofcurvature of the reflective surface when the reflective surface is aspherical surface, and a radius of curvature of a spherical surfaceafter the aspheric component is removed from the reflective surface ifthe reflective surface is an aspheric surface or a radius of curvaturebased on the curvature near the rotationally symmetrical axis of therotationally symmetrical aspheric surface that includes the asphericsurface), wherein the surface apexes of the six reflective surfaces arearranged in order of the fourth reflective surface, the secondreflective surface, the third reflective surface, the first reflectivesurface, the sixth reflective surface and the fifth reflective surfacein order from the object plane to the image plane along an optical axis.

When the surface interval becomes small, it is difficult to thicken themirror and to arrange members, such as a holding mechanism and a coolingmechanism. Considering this fact, a distance is 250 mm or longer, morepreferably 310 mm or longer, between the object plane and one of thesurface apexes of the six reflective surfaces, which one is the closestto the object plane.

A distance along the optical axis between the surface apex of the secondreflective surface and the surface apex of the fourth reflective surfaceis 5 mm or longer, preferably 10 mm or longer, more preferably 15 mm orlonger.

Preferably, with respect to an overall length of the projection opticalsystem or each of the six reflective surfaces, a surface apex isdefined, with respect to each of the six reflective surfaces, as one ofnodal points between an optical axis and an approximately sphericalsurface, which one is closest to a reflection point of the light on eachof the six reflective surfaces, the approximately spherical surfacehaving a center that is defined as a center of curvature of each of thesix reflective surfaces, and a radius that is defined as a radius ofcurvature of each of the six reflective surfaces, whereinLall/200<L24<Lall/10, more preferably, Lall /100<L24 and/or L24<Lall/18,is met, where L24 is an interval between the surface apex of the secondreflective surface and the surface apex of the fourth reflectivesurface, and Lall is a distance between the object plane and the imageplane along the optical axis.

In general, the sixth reflective surface having a large effectivediameter has a large thickness. Therefore, an interval between thesurface apex of the sixth reflective surface and the surface apex of thereflective surface closest to the sixth reflective surface along theoptical axis is 100 mm or longer, preferably 110 mm or longer, 115 mm orlonger. The instant system satisfies the above conditions and providesan easily configured system.

Preferably, Lall/20<L6<Lall/6, more preferably, Lall/12<L6 and/orL6<Lall/9, are met, where L6 is an interval between the surface apex ofthe sixth reflective surface and the surface apex of the reflectivesurface that is closest to the sixth reflective surface, and Lall is anoverall length of the projection optical system, as discussed.

Preferably, a surface apex is defined, with respect to each of the sixreflective surfaces, as one of nodal points between an optical axis andan approximately spherical surface, which one is closest to a reflectionpoint of the light on each of the six reflective surfaces, theapproximately spherical surface having a center that is defined as acenter of curvature of each of the six reflective surfaces, and a radiusthat is defined as a radius of curvature of each of the six reflectivesurfaces, wherein the surface apex of the third reflective surface islocated closer to the object plane than the surface apex of the sixthreflective surface.

Preferably, the third reflective surface is located closer to the objectplane along the optical path than the sixth reflective surface.

Preferably, the aperture stop is located between the first and secondreflective surfaces, and located apart from each of the first and secondreflective surfaces by Lst/10 or greater, more preferably, by Lst/5 orgreater, where Lst is an optical path length between the first andsecond reflective surfaces. This configuration can take advantage of thesecond reflective surface as means for introducing the light into thefourth reflective surface that is located apart from the optical axis.

Preferably, the six reflective surfaces are located between the objectplane and the image plane (or the six reflective surfaces are locatedbetween the object plane or the object-side plane including this objectplane and the image plane of the image-side plane including this imageplane). In addition, all the optical elements having optical powers arelocated between the object plane and the image plane in the catoptricprojection optical system.

At least one of the six reflective surfaces is an aspheric mirror havinga multilayer coating for reflecting the EUV light. Preferably, all ofthe six reflective surfaces are aspheric mirrors each having amultilayer coating for reflecting the EUV light.

Preferably, the light used for the projection optical system is EUVlight with a wavelength between the 10 nm and 20 nm, more preferably 13nm and 14 nm.

Preferably, the optical system at the object plane side isnon-telecentric. More preferably, the optical system at the image planeside is substantially telecentric.

An inventive exposure apparatus is a scanning exposure apparatus thatincludes an illumination optical system for illuminating a pattern on anobject plane using light from a light source, the above projectionoptical systems for projecting the pattern on the object plane onto animage plane in a reduced size, and a reflection mask located on theobject plane, and synchronously scans a mask stage and a wafer stage.

A description will be given of embodiments of the inventive catoptricprojection optical system with reference to FIGS. 1 to 3:

First Embodiment

A description will be given of a first embodiment of the presentinvention with reference to FIG. 1 and Table 1.

A catoptric projection optical system of the first embodiment includessix mirrors that includes, in order of reflecting the light from theobject plane MS, a first concave reflective surface M1, an aperturestop, a second convex reflective surface M2, a third convex reflectivesurface M3, a fourth concave surface M4, a fifth convex reflectivesurface M5 and a sixth concave reflective surface M6. The projectionoptical system forms an intermediate image IM between M4 and M5 alongthe optical path, and re-forms the intermediate image IM on the imageplane W using the remaining surfaces.

In FIG. 1, an alternate long and short dash line denotes an opticalaxis, which can also defined as a line that connects centers ofcurvature of the first to sixth reflective surfaces. Since eachreflective surface can be decentered and inclined for purposes ofaberrational corrections, the centers of curvature of the reflectivesurfaces do not always align with but can slightly offset from theoptical axis.

A distance between the object plane and the image plane along theoptical axis is referred to as an overall length, which is about1230.428 mm in the first embodiment.

MS is a reflection mask located at an object plane position, and W is awafer located at an image plane position. The inventive catoptricreduction projection optical system projects a reflection maskilluminated by the illumination optical system onto the wafer as theimage plane.

Table 1 shows details of the optical system shown in FIG. 1. A numericalaperture NA at the image side is 0.26, a magnification is ¼, and anobject point is 126 to 134 mm (while the image side has an arc fieldwith a width of 2 mm). The wave front aberration has a RMS of 7.2 mλ,and a static distortion range of 2.2 nm.

As discussed, an arrangement of the aperture stop between M1 and M2prevents shielding of the light from the object plane to M1, althoughthe object-side telecentricity is as small as 103 mrad. Since M2 and M3introduce the light from the aperture stop to M4 apart from the opticalaxis, a distance between M3 and M4 can be relatively short although themaximum incident angle is maintained to be 26.5°. This distance andsecond reflective surface having a convex shape provide an appropriatedivergence of the light on the fourth reflective surface M4. Morespecifically, the divergence of the light on the fourth surfaceintroduced from the object point with an object point of 130 mm, or adifference of the maximum value and the minimum value of distances fromthe optical axis on the light incident area on the fourth reflectivesurface, is 40.8 mm. This configuration reduces influences of the mirrorsurface's ripples, air bubbles in the mirror material, mirror'sdeformations, dust transfers, etc., and prevent deteriorations of theimaging performance. Therefore, the divergence of the light on M4becomes appropriate, while the maximum effective diameter is maintainedto be 560 mm.

Preferably, the aperture stop is located apart from the first and secondreflective surfaces M1 and M2 by a proper distance. In the instantembodiment, the aperture stop is located apart from the first reflectivesurface M1 by 0.668 Lst, and apart from the second reflective surface M1by 0.332 Lst, where Lst is an optical path length between the first andsecond reflective surfaces.

The third reflective surface is a surface that has the largest lightincident angle among the six reflective surfaces, which is 26.5°. Theincident angle distribution width is 2.5°. In particular, the smallincident angle distribution prevents reductions of the reflectance dueto the multilayer coating.

A distance is 333.7 mm between the object plane and the surface apex ofthe fourth reflective surface as a reflective surface closest to theobject plane, providing a sufficient front focus.

An interval is 46.1 mm between the surface apex of the second reflectivesurface and the surface apex of the fourth reflective surface, and aninterval is 120 mm between the surface apex of the sixth reflectivesurface and the surface apex of the first reflective surface as areflective surface that is closest to the sixth reflective surface. Thisconfiguration maintains the space, and facilitates arrangements ofvarious mechanisms, such as a driving mechanism and a cooling mechanism.L1/L2 is 0.92, which is sufficient to provide a reduced incident angle,and a sufficient front focus, where L1 is an interval between the objectplane and the surface apex of M4 that is the reflective surface closestto the object plane, and L2 is an interval between the surface apex ofM4 that is the reflective surface closest to the object plane and thesurface apex of the first reflective surface.

The convex surfaces of M2 and M3 enable the intermediate image to format a position apart from the mirror, and provide an appropriatedivergence of the light on the mirror surface. Thereby, thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Thisintermediate image is formed between 0.4×Lim and 0.6×Lim where Lim is anoptical path length between the fourth and fifth reflective surfaces.

TABLE 1 MIRROR RADIUS OF SURFACE NO. CURVATURE. INTERVAL M(MASK) ∞689.36580 M1 −686.96140 −212.64860 APERTURE ∞ −105.89250 STOP M2−1445.63800 232.34530 M3 741.90100 −278.44160 M4 611.25530 863.37590 M5308.44490 −378.74050 M6 455.92490 421.06450 W(WAFER) ∞ 0 ASPHERICCOEFFICIENT K A B C D E F G M1 −1.94563E+00  9.01813E−10 −2.12560E−14 4.33484E−19 −8.21909E−24  1.53956E−29  7.43682E−33 −1.71216E−37 M2 1.70220E+01 −1.73522E−09  9.43707E−14 −1.58260E−17  7.43820E−21−2.00780E−24  2.87488E−28 −1.64770E−32 M3  9.70635E−01 −2.08916E−09−6.16309E−14  9.35423E−18 −7.58825E−22  3.66802E−26 −9.77561E−31 1.10779E−35 M4 −2.62842E−02  3.96504E−11 −1.62737E−15  1.96140E−20−3.14125E−26 −1.93664E−30  2.01437E−35 −6.39960E−41 M5  1.64409E−01−1.00959E−09  1.23204E−12 −4.31799E−17  1.11326E−21  2.46707E−24−9.17157E−28  1.14419E−31 M6  3.26806E−02  3.66724E−11  2.36144E−16 8.37613E−22  3.80831E−26 −7.00742E−31 −6.44088E−36  3.83037E−40Second Embodiment

A description will be given of a second embodiment of the presentinvention with reference to FIG. 2 and Table 2. Unless otherwisespecified, this embodiment is similar to the first embodiment.

The overall length of the second embodiment is about 1211.432 mm. Anumerical aperture NA at the image side is 0.26, a magnification is ¼,and an object point is 126 to 134 mm (while the image side has an arcfield with a width of 2 mm). The wave front aberration has a RMS of 13mλ, and a static distortion range of 2.7 nm.

An arrangement of the aperture stop between M1 and M2 prevents shieldingof the light from the object plane to M1, although the object-sidetelecentricity is as small as 106.5 mrad. Since M2 and M3 introduce thelight from the aperture stop to M4 apart from the optical axis, adistance between M3 and M4 can be relatively short although the maximumincident angle is maintained to be 25.4°. This distance and secondreflective surface having a convex shape provide an appropriatedivergence of the light on the fourth reflective surface M4.

More specifically, the divergence of the light on the fourth surfaceintroduced from the object point of 130 mm is 60.3 mm. Thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Therefore,the divergence of the light on M4 becomes appropriate, while the maximumeffective diameter is maintained to be 560 mm.

Preferably, the aperture stop is located apart from the first and secondreflective surfaces M1 and M2 by a proper distance. In the instantembodiment, the aperture stop is located apart from the first reflectivesurface M1 by 0.685 Lst, and apart from the second reflective surface M1by 0.315 Lst, where Lst is an optical path length between the first andsecond reflective surfaces.

The third reflective surface is a surface that has the largest lightincident angle among the six reflective surfaces, which is 25.4°. Theincident angle distribution width is 3.3°. In particular, the smallincident angle distribution prevents reductions of the reflectance dueto the multilayer coating.

A distance is 329.2 mm between the object plane and the surface apex ofthe fourth reflective surface as a reflective surface closest to theobject plane, providing a sufficient front focus.

An interval is 30.5 mm between the surface apex of the second reflectivesurface and the surface apex of the fourth reflective surface, and aninterval is 120 mm between the surface apex of the sixth reflectivesurface and the surface apex of the first reflective surface as areflective surface that is closest to the sixth reflective surface. Thisconfiguration maintains the space, and facilitates arrangements ofvarious mechanisms, such as a driving mechanism and a cooling mechanism.L1/L2 is 0.92, which is sufficient to provide a reduced incident angle,and a sufficient front focus, where L1 is an interval between the objectplane and the surface apex of M4 that is the reflective surface closestto the object plane, and L2 is an interval between the surface apex ofM4 that is the reflective surface closest to the object plane and thesurface apex of the first reflective surface.

The convex surfaces of M2 and M3 enable the intermediate image to format a position apart from the mirror, and provide an appropriatedivergence of the light on the mirror surface. Thereby, thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Thisintermediate image is formed between 0.4×Lim and 0.6×Lim where Lim is anoptical path length between the fourth and fifth reflective surfaces.

TABLE 2 MIRROR RADIUS OF SURFACE NO. CURVATURE. INTERVAL M(MASK) ∞664.18850 M1 −666.17780 −207.90840 APERTURE ∞ −95.59360 STOP M2−828.86400 244.07190 M3 1387.89490 −275.54020 M4 644.72050 839.61750 M5307.71430 −384.64710 M6 464.62120 437.24330 W(WAFER) ∞ 0 ASPHERICCOEFFICIENT K A B C D E F G M1 −2.35596E+00  7.84893E−10 −2.72820E−14 6.48140E−19 −1.35529E−23 −3.31779E−28  6.02190E−32 −2.04791E−36 M2 2.35964E+01  5.65519E−09  1.59335E−13 −3.76810E−19  5.20728E−21−2.40912E−24  5.28091E−28 −4.60368E−32 M3 −2.68273E+00 −7.50969E−10 1.14233E−14 −2.73120E−19 −4.49599E−24  7.50420E−28 −2.40526E−32 2.62659E−37 M4  1.90138E−02 −1.71190E−11 −3.09570E−16  3.72374E−21 1.91792E−26 −1.09103E−30  1.12604E−35 −3.85577E−41 M5 −5.12215E−01 4.33837E−10  1.44464E−12 −6.30460E−17  9.20270E−21 −1.50781E−24−2.03544E−28  9.70776E−32 M6  2.23465E−02  3.21944E−11  2.11841E−16 1.00378E−21  3.48085E−26  2.88806E−30  1.19494E−34 −1.77676E−39Third Embodiment

A description will be given of a third embodiment of the presentinvention with reference to FIG. 3 and Table 3. Unless otherwisespecified, this embodiment is similar to the first and secondembodiments.

The overall length of the third embodiment is about 1206.092 mm. Anumerical aperture NA at the image side is 0.27, a magnification is ¼,and an object point is 126 to 134 mm (while the image side has an arcfield with a width of 2 mm). The wave front aberration has a RMS of 14.4mλ, and a static distortion range of 2.3 nm. 15 An arrangement of theaperture stop between M1 and M2 prevents shielding of the light from theobject plane to M1, although the object-side telecentricity is as smallas 103 mrad. Since M2 and M3 introduce the light from the aperture stopto M4 apart from the optical axis, a distance between M3 and M4 can berelatively short although the maximum incident angle is maintained to be26.5°. This distance and second reflective surface having a convex shapeprovide an appropriate divergence of the light on the fourth reflectivesurface M4.

More specifically, the divergence of the light on the fourth surfaceintroduced from the object point of 130 mm is 60.3 mm. Thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Therefore,the divergence of the light on M4 becomes appropriate, while the maximumeffective diameter is maintained to be 560 mm.

Preferably, the aperture stop is located apart from the first and secondreflective surfaces M1 and M2 by a proper distance. In the instantembodiment, the aperture stop is located apart from the first reflectivesurface M1 by 0.657 Lst, and apart from the second reflective surface Mlby 0.343 Lst, where Lst is an optical path length between the first andsecond reflective surfaces.

The third reflective surface is a surface that has the largest lightincident angle among the six reflective surfaces, which is 26.5°. Theincident angle distribution width is 2.2°. In particular, the smallincident angle distribution prevents reductions of the reflectance dueto the multilayer coating.

A distance is 341.5 mm between the object plane and the surface apex ofthe fourth reflective surface as a reflective surface closest to theobject plane, providing a sufficient front focus. An interval is 20 mmbetween the surface apex of the second reflective surface and thesurface apex of the fourth reflective surface, and an interval is 120 mmbetween the surface apex of the sixth reflective surface and the surfaceapex of the first reflective surface as a reflective surface that isclosest to the sixth reflective surface. This configuration maintainsthe space, and facilitates arrangements of various mechanisms, such as adriving mechanism and a cooling mechanism. L1/L2 is 0.97, which issufficient to provide a reduced incident angle, and a sufficient frontfocus, where L1 is an interval between the object plane and the surfaceapex of M4 that is the reflective surface closest to the object plane,and L2 is an interval between the surface apex of M4 that is thereflective surface closest to the object plane and the surface apex ofthe first reflective surface.

The convex surfaces of M2 and M3 enable the intermediate image to format a position apart from the mirror, and provide an appropriatedivergence of the light on the mirror surface. Thereby, thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Thisintermediate image is formed between 0.4×Lim and 0.6×Lim where Lim is anoptical path length between the fourth and fifth reflective surfaces.

TABLE 3 MIRROR RADIUS OF SURFACE NO. CURVATURE. INTERVAL M(MASK) ∞694.84276 M1 −716.24760 −219.13081 APERTURE ∞ −114.33524 STOP M2−1550.00000 241.59945 M3 721.61644 −261.49378 M4 586.31403 817.09236 M5296.69119 −353.73198 M6 430.15510 401.24950 W(WAFER) ∞ 0 ASPHERICCOEFFICIENT K A B C D E F G M1 −1.92463E+00  8.59913E−10 −2.03466E−14 4.62151E−19 −8.54869E−24 −2.76473E−28  3.16917E−32 −7.82844E−37 M2−2.88259E−01 −2.29465E−09  8.45007E−14 −1.61951E−17  7.60003E−21−1.92909E−24  2.47481E−28 −1.19995E−32 M3  1.17791E+00 −1.85904E−09−8.39806E−14  1.11840E−17 −8.47167E−22  3.85393E−26 −9.68293E−31 1.03379E−35 M4 −3.00880E−02  2.80737E−11 −1.37915E−15  2.11145E−20−9.66767E−26 −1.37500E−30  1.93512E−35 −6.99842E−41 M5  3.12023E−01−1.03445E−09  1.58463E−12 −8.00021E−17  2.19921E−21  8.72805E−24−3.83620E−27  5.39287E−31 M6  3.02209E−02  4.87630E−11  3.48587E−16 5.93966E−22  1.37823E−25 −5.78066E−30  1.46588E−34 −1.55827E−39Fourth Embodiment

A description will be given of a fourth embodiment of the presentinvention with reference to FIG. 4 and Table 4. Unless otherwisespecified, this embodiment is similar to the first, second and thirdembodiments.

The overall length of the third embodiment is about 1252.384 mm. Anumerical aperture NA at the image side is 0.25, a magnification is ¼,and an object point is 119 to 139 mm (while the image side has an arcfield with a width of 5 mm). The wave front aberration has a RMS of 17.4mλ, and a static distortion range of 2.7 nm.

An arrangement of the aperture stop between M1 and M2 prevents shieldingof the light from the object plane to M1, although the object-sidetelecentricity is as small as 103 mrad. Since M2 and M3 introduce thelight from the aperture stop to M4 apart from the optical axis, adistance between M3 and M4 can be relatively short although the maximumincident angle is maintained to be 27°. This distance and secondreflective surface having a convex shape provide an appropriatedivergence of the light on the fourth reflective surface M4.

More specifically, the divergence of the light on the fourth surfaceintroduced from the object point of 129 mm is 45.2 mm. Thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Therefore,the divergence of the light on M4 becomes appropriate, while the maximumeffective diameter is maintained to be 585 mm.

Preferably, the aperture stop is located apart from the first and secondreflective surfaces M1 and M2 by a proper distance. In the instantembodiment, the aperture stop is located apart from the first reflectivesurface M1 by 0.654 Lst, and apart from the second reflective surface M1by 0.346 Lst, where Lst is an optical path length between the first andsecond reflective surfaces.

The third reflective surface is a surface that has the largest lightincident angle among the six reflective surfaces, which is 27°. Theincident angle distribution width is 4.4°. In particular, the smallincident angle distribution prevents reductions of the reflectance dueto the multilayer coating.

A distance is 321.9 mm between the object plane and the surface apex ofthe fourth reflective surface as a reflective surface closest to theobject plane, providing a sufficient front focus. An interval is 61.4 mmbetween the surface apex of the second reflective surface and thesurface apex of the fourth reflective surface, and an interval is 116 mmbetween the surface apex of the sixth reflective surface and the surfaceapex of the first reflective surface as a reflective surface that isclosest to the sixth reflective surface. This configuration maintainsthe space, and facilitates arrangements of various mechanisms, such as adriving mechanism and a cooling mechanism. L1/L2 is 0.830, which issufficient to provide a reduced incident angle, and a sufficient frontfocus, where L1 is an interval between the object plane and the surfaceapex of M4 that is the reflective surface closest to the object plane,and L2 is an interval between the surface apex of M4 that is thereflective surface closest to the object plane and the surface apex ofthe first reflective surface. 5 The convex surfaces of M2 and M3 enablethe intermediate image to form at a position apart from the mirror, andprovide an appropriate divergence of the light on the mirror surface.Thereby, this configuration reduces influences of the mirror surface'sripples, air bubbles in 10 the mirror material, mirror's deformations,dust transfers, etc., and prevent deteriorations of the imagingperformance. This intermediate image is formed between 0.4×Lim and0.6×Lim where Lim is an optical path length between the fourth and fifthreflective surfaces.

TABLE 4 MIRROR RADIUS OF SURFACE NO. CURVATURE. INTERVAL M(MASK) ∞709.84800 M1 −706.52400 −213.48700 APERTURE ∞ −112.96900 STOP M2−1500.00000 224.72300 M3 700.00000 −286.16900 M4 620.33200 888.54800 M5323.10900 −384.64500 M6 462.76200 426.53500 W(WAFER) ∞ 0 ASPHERICCOEFFICIENT K A B C D E F G M1 −1.74858E+00  9.23055E−10 −2.11112E−14 6.26883E−19 −5.13981E−23  4.93156E−27 −2.75614E−31  6.38279E−36 M2 1.34777E+01 −2.39736E−09  5.46835E−14  1.12216E−17 −9.34360E−21 3.73946E−24 −7.55884E−28  6.15488E−32 M3  1.03737E+00 −2.27691E−09−8.91005E−14  1.17202E−17 −8.81984E−22  4.05103E−26 −1.04360E−30 1.15546E−35 M4 −2.54829E−02  8.94333E−11 −2.34462E−15  2.18505E−20 1.61745E−26 −2.39650E−30  2.05917E−35 −5.87149E−41 M5  3.42381E−01−7.90071E−10  1.15783E−12 −8.04750E−17  5.00002E−20 −2.80048E−23 8.24008E−27 −9.64674E−31 M6  3.48259E−02  3.52475E−11  2.12796E−16 1.57268E−21 −5.50112E−27  1.06617E−31  8.17667E−36 −1.98359E−40Fifth Embodiment

A description will be given of a fifth embodiment of the presentinvention with reference to FIG. 5 and Table 5. Unless otherwisespecified, this embodiment is similar to the first to fourthembodiments.

The overall length of the third embodiment is about 1267.046 mm. Anumerical aperture NA at the image side is 0.237, a magnification is ¼,and an object point is 117.5 to 140.5 mm (while the image side has anarc field with a width of 5.75 mm). The wave front aberration has a RMSof 17.6 mλ, and a static distortion range of 1.5 nm.

An arrangement of the aperture stop between M1 and M2 prevents shieldingof the light from the object plane to M1, although the object-sidetelecentricity is as small as 103 mrad. Since M2 and M3 introduce thelight from the aperture stop to M4 apart from the optical axis, adistance between M3 and M4 can be relatively short although the maximumincident angle is maintained to be 27°. This distance and secondreflective surface having a convex shape provide an appropriatedivergence of the light on the fourth reflective surface M4.

More specifically, the divergence of the light on the fourth surfaceintroduced from the object point of 129 mm is 45.5 mm. Thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Therefore,the divergence of the light on M4 becomes appropriate, while the maximumeffective diameter is maintained to be 585 mm.

Preferably, the aperture stop is located apart from the first and secondreflective surfaces M1 and M2 by a proper distance. In the instantembodiment, the aperture stop is located apart from the first reflectivesurface M1 by 0.656 Lst, and apart from the second reflective surface M1by 0.344 Lst, where Lst is an optical path length between the first andsecond reflective surfaces.

The third reflective surface is a surface that has the largest lightincident angle among the six reflective surfaces, which is 27°. Theincident angle distribution width is 4.8°. In particular, the smallincident angle distribution prevents reductions of the reflectance dueto the multilayer coating.

A distance is 336.6 mm between the object plane and the surface apex ofthe fourth reflective surface as a reflective surface closest to theobject plane, providing a sufficient front focus. An interval is 52.6 mmbetween the surface apex of the second reflective surface and thesurface apex of the fourth reflective surface, and an interval is 116 mmbetween the surface apex of the sixth reflective surface and the surfaceapex of the first reflective surface as a reflective surface that isclosest to the sixth reflective surface. This configuration maintainsthe space, and facilitates arrangements of various mechanisms, such as adriving mechanism and a cooling mechanism. L1/L2 is 0.888, which issufficient to provide a reduced incident angle, and a sufficient frontfocus, where L1 is an interval between the object plane and the surfaceapex of M4 that is the reflective surface closest to the object plane,and L2 is an interval between the surface apex of M4 that is thereflective surface closest to the object plane and the surface apex ofthe first reflective surface.

The convex surfaces of M2 and M3 enable the intermediate image to format a position apart from the mirror, and provide an appropriatedivergence of the light on the mirror surface. Thereby, thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Thisintermediate image is formed between 0.4×Lim and 0.6×Lim where Lim is anoptical path length between the fourth and fifth reflective surfaces.

TABLE 5 MIRROR RADIUS OF SURFACE NO. CURVATURE. INTERVAL M(MASK) ∞715.81900 M1 −716.12100 −214.26900 APERTURE ∞ −112.38900 STOP M2−1500.00000 230.03000 M3 700.00000 −282.61400 M4 618.19100 888.36900 M5328.80000 −393.12700 M6 471.62300 435.12700 W(WAFER) ∞ 0 ASPHERICCOEFFICIENT K A B C D E F G M1 −1.71111E+00  9.12114E−10 −2.08677E−14 5.53638E−19 −3.84470E−23  3.63512E−27 −1.99427E−31  4.40002E−36 M2 1.89149E+01 −2.33866E−09  5.14450E−14  1.55157E−17 −1.21994E−20 4.85447E−24 −9.95274E−28  8.27121E−32 M3  1.14144E+00 −2.26421E−09−9.14399E−14  1.17075E−17 −8.65006E−22  3.90349E−26 −9.88059E−31 1.07411E−35 M4 −2.53894E−02  7.08547E−11 −2.04123E−15  1.92894E−20 1.31597E−26 −2.17749E−30  1.91061E−35 −5.55658E−41 M5  4.85584E−01−1.05702E−09  1.10386E−12 −3.98425E−17  1.35295E−20 −8.97234E−24 2.59639E−27 −2.44935E−31 M6  1.86590E−02  5.28168E−11  2.79824E−16 1.75250E−21  9.21551E−27 −1.04462E−30  5.23515E−35 −8.18886E−40Sixth Embodiment

A description will be given of a sixth embodiment of the presentinvention with reference to FIG. 6 and Table 6. Unless otherwisespecified, this embodiment is similar to the first to fifth embodiments.

The overall length of the third embodiment is about 1268.513 mm. Anumerical aperture NA at the image side is 0.23, a magnification is ¼,and an object point is 117 to 141 mm (while the image side has an arcfield with a width of 6 mm). The wave front aberration has a RMS of 17.0mλ, and a static distortion range of 3.0 nm.

An arrangement of the aperture stop between M1 and M2 prevents shieldingof the light from the object plane to M1, although the object-sidetelecentricity is as small as 103 mrad. Since M2 and M3 introduce thelight from the aperture stop to M4 apart from the optical axis, adistance between M3 and M4 can be relatively short although the maximumincident angle is maintained to be 27°. This distance and secondreflective surface having a convex shape provide an appropriatedivergence of the light on the fourth reflective surface M4.

More specifically, the divergence of the light on the fourth surfaceintroduced from the object point of 129 mm is 45.0 mm. Thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Therefore,the divergence of the light on M4 becomes appropriate, while the maximumeffective diameter is maintained to be 585 mm.

Preferably, the aperture stop is located apart from the first and secondreflective surfaces M1 and M2 by a proper distance. In the instantembodiment, the aperture stop is located apart from the first reflectivesurface M1 by 0.659 Lst, and apart from the second reflective surface M1by 0.341 Lst, where Lst is an optical path length between the first andsecond reflective surfaces.

The third reflective surface is a surface that has the largest lightincident angle among the six reflective surfaces, which is 27°. Theincident angle distribution width is 4.9°. In particular, the smallincident angle distribution prevents reductions of the reflectance dueto the multilayer coating.

A distance is 340.8 mm between the object plane and the surface apex ofthe fourth reflective surface as a reflective surface closest to theobject plane, providing a sufficient front focus. An interval is 50.7 mmbetween the surface apex of the second reflective surface and thesurface apex of the fourth reflective surface, and an interval is 116 mmbetween the surface apex of the sixth reflective surface and the surfaceapex of the first reflective surface as a reflective surface that isclosest to the sixth reflective surface. This configuration maintainsthe space, and facilitates arrangements of various mechanisms, such as adriving mechanism and a cooling mechanism. L1/L2 is 0.907, which issufficient to provide a reduced incident angle, and a sufficient frontfocus, where L1 is an interval between the object plane and the surfaceapex of M4 that is the reflective surface closest to the object plane,and L2 is an interval between the surface apex of M4 that is thereflective surface closest to the object plane and the surface apex ofthe first reflective surface.

The convex surfaces of M2 and M3 enable the intermediate image to format a position apart from the mirror, and provide an appropriatedivergence of the light on the mirror surface. Thereby, thisconfiguration reduces influences of the mirror surface's ripples, airbubbles in the mirror material, mirror's deformations, dust transfers,etc., and prevent deteriorations of the imaging performance. Thisintermediate image is formed between 0.4×Lim and 0.6×Lim where Lim is anoptical path length between the fourth and fifth reflective surfaces.

TABLE 6 MIRROR RADIUS OF SURFACE NO. CURVATURE. INTERVAL M(MASK) ∞716.58300 M1 −717.61400 −214.40900 APERTURE ∞ −110.70300 STOP M2−1500.00000 231.76600 M3 700.00000 −282.46700 M4 617.60500 885.74300 M5329.40000 −393.92900 M6 472.31400 435.92900 W(WAFER) ∞ 0 ASPHERICCOEFFICIENT K A B C D E F G M1 −1.74713E+00  8.94795E−10 −2.08679E−14 5.70246E−19 −4.28929E−23  4.19048E−27 −2.30821E−31  5.03221E−36 M2 2.23191E+01 −2.26707E−09  5.76002E−14  1.44486E−17 −1.25019E−20 5.24338E−24 −1.11928E−27  9.61434E−32 M3  1.15153E+00 −2.24785E−09−9.21410E−14  1.17375E−17 −8.63698E−22  3.88096E−26 −9.77901E−31 1.05773E−35 M4 −2.50634E−02  6.85004E−11 −1.99900E−15  1.90076E−20 1.20330E−26 −2.14854E−30  1.89674E−35 −5.54395E−41 M5  4.77515E−01−1.01282E−09  1.10973E−12 −4.38480E−17  2.22619E−20 −1.55145E−23 4.68958E−27 −4.86211E−31 M6  1.88278E−02  5.25050E−11  2.76702E−16 1.46126E−21  4.71248E−26 −3.74731E−30  1.49207E−34 −2.17320E−39Seventh Embodiment

A description will now be given of an exposure apparatus 200 thatincludes the projection optical system 100, 100A, 100B, 100C, 100D or100E of the first to sixth embodiments, with reference to FIG. 7. Here,FIG. 7 is a schematic structure of the exposure apparatus 200 accordingto one aspect of the present invention.

The exposure apparatus 200 uses EUV light (with a wavelength, forexample, of 13.5 nm) as illumination light for exposure, and exposesonto the plate 240 a circuit pattern created on a mask 220, for example,in a step-and-scan manner or step-and-repeat manner. This exposureapparatus is suitable for a lithography process less than submicron orquarter micron, and the present embodiment uses the step-and-scanexposure apparatus (also referred to as a “scanner”) as an example. The“step-and-scan manner”, as used herein, is an exposure method thatexposes a mask pattern onto a wafer by continuously scanning the waferrelative to the mask, and by moving, after a shot of exposure, the waferstepwise to the next exposure area to be shot. The “step-and-repeatmanner” is another mode of exposure method that moves a wafer stepwiseto an exposure area for the next shot every shot of cell projection ontothe wafer.

Referring to FIG. 7, the exposure apparatus 200 includes an illuminationapparatus 210 for illuminating the mask 220 using the light from a lightsource, a mask stage 225 that supports the mask 220, a projectionoptical system 230 (100, or the like) for introducing the light from themask 220 into an object 240 to be exposed, a wafer stage 245 thatsupports the object 240, an alignment detection mechanism 250, and afocus position detection mechanism 260. Although FIG. 7 shows afour-mirror catoptric projection optical system from the mask to theobject (wafer) for simplicity purposes, the number of reflectivesurfaces in the catoptric projection optical system is preferably six asin the above first to sixth embodiments. Of course, the number ofmirrors is variable within a scope of the present invention.

At least the optical path through which the EUV light travels ispreferably be maintained in a vacuum atmosphere VC, although not shownin FIG. 7, since the EUV light has low transmittance for air and theresidue gas (such as polymer organic gas) causes contaminations.

The illumination apparatus 210 uses the EUV light (with a wavelength,for example, of 13.4 nm) of an arc shape corresponding to an arc fieldof the projection optical system 230 to illuminate the mask 220, andincludes an EUV light source 212 and an illumination optical system 214.

The EUV light source 210 uses, for example, a laser plasma light source.The laser plasma light source irradiates a highly intensified pulselaser beam to a target material put in vacuum, thus generatinghigh-temperature plasma for use as EUV light with a wavelength of about13 nm emitted from this. The target material may use a metallic thinfilm, inert gas, and droplets, etc. The pulse laser preferably has highrepetitive frequency, e.g., usually several kHz, for increased averageintensity of the emitted EUV light.

The illumination optical system 214 includes a condenser mirror 214 a,and an optical integrator 214 b. The condenser mirror 214 a serves tocollect the EUV light that is isotropically irradiated from the laserplasma. The optical integrator 214 b serves to uniformly illuminate themask 220 with a predetermined NA. The illumination optical system 214further includes an aperture 214 c to limit an illumination area to anarc shape at a position conjugate with the mask 220. The illuminationoptical system 214 may further include a cooling apparatus for coolingth e optical elements, such as the condenser mirror 214 a and theoptical integrator 214 b. The cooling apparatus cools the condensermirror 214 a and optical integrator 214 b, and prevents deformation dueto the thermal expansion for excellent imaging performance.

The mask 220 is a reflection mask that forms a circuit pattern or imageto be transferred, and supported and driven by the mask stage 225. Thediffracted light from the mask 220 is reflected by the projectionoptical system 230 (100 etc.) discussed in the first to sixthembodiments and projected onto the object 240. The mask 220 and theobject 240 are arranged optically conjugate with each other. Theexposure apparatus 200 is a step-and-scan exposure apparatus, andprojects a reduced size of the pattern on the mask 220 on the object 240by scanning the mask 220 and the object 240.

The mask stage 225 supports the mask 220 and is connected to a movingmechanism (not shown). The mask stage 225 may use any structure known inthe art. A moving mechanism (not shown) may include a linear motor etc.,and drives the mask stage 225 at least in an X direction and moves themask 220. The exposure apparatus 200 synchronously scans the mask 220 orthe object 240.

The projection optical system 230 uses plural multilayer mirrors 230 ato project a reduced size of a pattern formed on the mask 220 onto theobject 240. The number of mirrors 230 a is six or more. For wideexposure area with the small number of mirrors, the mask 220 and object240 are simultaneously scanned to transfer a wide area that is anarc-shaped area or ring field apart from the optical axis by apredetermined distance. The projection optical system 230 has a NA ofabout 0.2 to 0.3. A cooling apparatus can cool an optical element in theprojection optical system 230 such as the mirror 230 a. The coolingapparatus cools the mirror 230 a, and prevents deformation due to thethermal expansion for excellent imaging performance.

The instant embodiment uses a wafer as the object 240 to be exposed, butit may include a liquid crystal plate and a wide range of other objectsto be exposed. A photoresist is applied onto the object 240.

The wafer stage 245 holds the object 240 by a wafer chuck 245 a. Thewafer stage 245 moves the object 240, for example, using a linear stagein XYZ directions. The mask 220 and the object 240 are synchronouslyscanned. The positions of the mask stage 225 and wafer stage 245 aremonitored, for example, by a laser interferometer, and driven at aconstant speed ratio.

The alignment detection mechanism 250 measures a positional relationshipbetween the position of the mask 220 and the optical axis of theprojection optical system 230, and a positional relationship between theposition of the object 240 and the optical axis of the projectionoptical system 230, and sets positions and angles of the mask stage 225and the wafer stage 245 so that a projected image of the mask 220 may bepositioned in place on the object 240.

A focus position detection mechanism 260 measures a focus position onthe object 240 surface, and its control over positions and angles of thewafer stage 245 always maintain the object 240 surface at an imagingposition of the projection optical system 230 during exposure.

In exposure, the EUV light emitted from the illumination apparatus 210illuminates the mask 220, and images a pattern formed on the mask 220onto the object 240 surface. The instant embodiment uses an arc or ringshaped image plane, scans the mask 220 and object 240 at a speed ratiocorresponding to a reduction ratio to expose the entire surface of themask 220.

As the optical performance is sensitive to a surface shape of theoptical element in the projection optical system in the exposureapparatus, the cooling apparatus is often used to cool an opticalelement in the projection optical system, in particular, an opticalelement at a mask side that receives much light intensity. Of course, itmay be used for the illumination optical system. In particular, thereflective optical element closest to a light source receives a largeamount of light among the optical elements, generates the large absorbedheat value inevitably, and the absorbed heat value deforms the shape ofthe optical element. In order to prevent these problems, the abovecooling apparatus prevents the temperature rise due to absorption of alarge amount of light, and reduces a temperature difference in an ashape change of the optical element.

Referring to FIGS. 8 and 9, a description will now be given of anembodiment of a device fabricating method using the above exposureapparatus 200. FIG. 8 is a flowchart for explaining a fabrication ofdevices (i.e., semiconductor chips such as IC and LSI, LCDs, CCDs, etc.)Here, a description will be given of a fabrication of a semiconductorchip as an example. Step 1 (circuit design) designs a semiconductordevice circuit. Step 2 (mask fabrication) forms a mask having a designedcircuit pattern. Step 3 (wafer preparation) manufactures a wafer usingmaterials such as silicon. Step 4 (wafer process), which is referred toas a pretreatment, forms actual circuitry on the wafer throughphotolithography using the mask and wafer. Step 5 (assembly), which isalso referred to as a post treatment, forms into a semiconductor chipthe wafer formed in Step 4 and includes an assembly step (e.g., dicing,bonding), a packaging step (chip sealing), and the like. Step 6(inspection) performs various tests for the semiconductor device made inStep 5, such as a validity test and a durability test. Through thesesteps, a semiconductor device is finished and shipped (Step 7).

FIG. 9 is a detailed flowchart of the wafer process in step 4. Step 11(oxidation) oxidizes the wafer's surface. Step 12 (CVD) forms aninsulating film on the wafer's surface. Step 13 (electrode formation)forms electrodes on the wafer by vapor disposition and the like. Step 14(ion implantation) implants ions into the wafer. Step 15 (resistprocess) applies a photosensitive material onto the wafer. Step 16(exposure) uses the exposure apparatus 200 to expose a circuit patternon the mask onto the wafer. Step 17 (development) develops the exposedwafer. Step 18 (etching) etches parts other than a developed resistimage. Step 19 (resist stripping) removes disused resist after etching.These steps are repeated, and multilayer circuit patterns are formed onthe wafer. The device fabrication method of this embodiment maymanufacture higher quality devices than the conventional one. Thus, thedevice fabrication method using the exposure apparatus 200, and thedevices as finished goods also constitute one aspect of the presentinvention.

Further, the present invention is not limited to these preferredembodiments, and various variations and modifications may be madewithout departing from the scope of the present invention. For example,the cooling apparatus is applicable to an optical element including amask and a wafer for non-EUV, ultraviolet light with a wavelength of 200nm or smaller, such as ArF excimer laser and F₂ excimer laser.

Thus, the instant embodiments can provide a projection optical systemthat has a reduced incident angle and a reduced mirror's maximumeffective diameter, and maintains an appropriate divergence of the lighton the mirror, an exposure apparatus having the projection opticalsystem, and a device fabrication method using the exposure apparatus.

This application claims a foreign priority based on Japanese PatentApplications Nos. 2003-310118, filed on Sep. 2, 2003 and 2003-316940,filed on Sep. 9, 2003, which is hereby incorporated by reference herein.

1. A projection optical for projecting a pattern on an object plane ontoan image plane in a reduced size, said projection optical systemcomprising six reflective surfaces that include, in order of reflectinglight from the object plane, a first reflective surface, a second convexreflective surface, a third convex reflective surface, a fourthreflective surface, a fifth reflective surface and a sixth reflectivesurface, and an aperture stop along an optical path between the firstand second reflective surfaces, wherein the six reflective surfaces arearranged so that radii of curvature of these six reflective surfaces aresubstantially aligned with a certain optical axis, wherein a surfaceapex is defined, with respect to each of the six reflective surfaces, asone of nodal points between an optical axis and an approximatelyspherical surface, which one is closest to a reflection point of thelight on each of the six reflective surfaces, the approximatelyspherical surface having a center that is defined as a center ofcurvature of each of the six reflective surfaces, and a radius that isdefined as a radius of curvature of each of the six reflective surfaces,and wherein the following condition is met, where L1 is an intervalbetween the object plane and the surface apex that is the closest to theobject plane, and L2 is an interval between the surface apex of thefirst reflective surface and the surface apex that is the closest to theobject plane: ${0.75 < \frac{L1}{L2} < 1.25},$ wherein an intermediateimage of the pattern on the object plane is formed between two adjacentreflective surfaces along the optical path of the light among the sixreflective surfaces, and located apart from each of the two adjacentreflective surfaces by Lim×0.35 or greater, where Lim is an optical pathlength between the two adjacent reflective surfaces.
 2. An exposureapparatus comprising: an illumination optical system for illuminating apattern on an object plane using light from a light source; and aprojection optical system according to claim 1 for projecting thepattern on the object plane onto an image plane in a reduced size.
 3. Adevice fabricating method comprising the steps of: exposing an objectusing the exposure apparatus according to claim 2; and developing theobject that has been exposed.